sh: Fix race condition with signals and wait or set -T.

The change in r238888 was incomplete. It was still possible for a trapped
signal to arrive before the shell went to sleep (sigsuspend()) because a
check was missing or because the signal arrived before in_waitcmd was set.

On SMP, this bug sometimes caused the builtins/wait4.0 test to take 1 second
to execute; it then might or might not fail. On UP, the test almost always
failed.
Better conformance to style(9) and organizational cleanup.
No functional changes.

sys/i386/xen/mp_machdep.c:
	Remove extra newlines.

	Group externs, forward delarations, local types, and pcpu data.

	Wrap at 80 columns.

	Use parens in return statements.

	Tab indent members of array initializers.

MFC after:	2 weeks